Move state reshape in gates

[stavros11]

Moves the state reshape required for non-custom backends inside the gates instead of the circuit. This fixes some compatibility issues between gates of different backends and makes testing easier.

It also fixes #337 since the tensor_shape and flat_shape attributes are no longer required during circuit creation. For this point I also added a test that initializes a QFT for n=100 qubits and checks that the depth is 2n and the number of gates n^2/2 + n.

[codecov]

Codecov Report

Merging #340 (27cdec4) into master (8502ee6) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #340   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           74        74           
  Lines        12092     12082   -10     
=========================================
- Hits         12092     12082   -10     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/qibo/core/circuit.py	100.00% <ø> (ø)
src/qibo/tests_new/test_core_gates.py	100.00% <ø> (ø)
src/qibo/core/gates.py	100.00% <100.00%> (ø)
src/qibo/tests/test_density_matrix.py	100.00% <100.00%> (ø)
src/qibo/tests/test_qft.py	100.00% <100.00%> (ø)
src/qibo/tests_new/test_core_circuit_features.py	100.00% <100.00%> (ø)
...c/qibo/tests_new/test_core_gates_density_matrix.py	100.00% <100.00%> (ø)

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[scarrazza]

@stavros11 thanks for this implementation and fix, looks good!